1. Field of the Invention
The present invention relates to methods for producing magnetoresistive elements. The invention also relates to magnetoresistive elements, as well as magnetic heads, magnetic memories and magnetic recording devices, which are magnetic devices using the same.
2. Description of the Related Art
With the recent developments in advanced communication networks, there is a demand for devices capable of handling a large volume of information at high speeds. For example, as large-capacity, high-speed devices, expectations are growing for magnetic heads and magnetic memories (MRAMs) that utilize the tunneling magnetoresistance effect (TMR effect).
The TMR effect is the phenomenon in which the resistance value varies depending on a relative angle between the magnetization directions of a pair of magnetic layers laminated with a tunnel insulating layer interposed therebetween. Magnetoresistive elements (TMR elements) utilizing this phenomenon have a ratio of change in the magnetoresistance (MR ratio) in a minute magnetic field that is by far larger than elements utilizing the anisotropic magnetoresistance effect (AMR effect) or the giant magnetoresistance effect (GMR effect). Therefore, extensive developments are under way to apply TMR elements to next-generation magnetic heads and MRAMs.
Each of the layers constituting a TMR element is extremely thin, and is on the order of several nm to several tens of nm. In order to achieve a TMR element with excellent magnetoresistance properties (MR properties), it is important to control these layers. Particularly, the state of a tunnel insulating layer is considered to have a significant effect on the MR properties of the element.
For example, in the case of using a TMR element for a device such as a magnetic head, it is preferable to realize a large MR ratio and minimize (e.g., 10 Ω·μm2 or less) the junction resistance value (resistance value per unit area when a current is supplied in a direction perpendicular to the film plane direction of the element). When the junction resistance value is small, it is possible to suppress, for example, the generation of shot noise, which is the phenomenon of electrons being transmitted randomly through the tunnel insulating layer (shot noise causes a reduction in the S/N (signal-to-noise ratio) of the element). The junction resistance value can be reduced by, for example, decreasing the thickness of the tunnel insulating layer. However, simply decreasing the thickness of the tunnel insulating layer possibly may reduce the resulting MR ratio. In general, the interface between the tunnel insulating layer and a magnetic layer in contact therewith is not completely smooth, exhibiting a roughness at the atomic level in the sub-nanometer to several nanometer range. That is, regions in which the thickness is locally large and regions in which the thickness is locally small are present in the tunnel insulating layer. Therefore, there is the possibility that with a decrease in the thickness of the tunnel insulating layer, a leakage current may be generated in the region in which the thickness is locally small. Since a leakage current does not contribute to the MR effect, this causes a reduction in the MR ratio, although the apparent junction resistance value is reduced.
In addition, TMR elements have the problem that the resulting MR ratio decreases with an increase in the applied bias voltage. In the case of a MRAM using TMR elements, for example, a bias voltage of about 400 mV generally is applied. In this state, the resulting MR ratio is about half of that in the state in which no bias voltage is applied. Such “bias voltage dependence of the MR ratio” is considered to be attributed to, for example, lattice defects in the tunnel insulating layer, impurities contained in the tunnel insulating layer, elementary excitations on the interface between the tunnel insulating layer and the magnetic layer and mismatches in the band structure. Among them, lattice defects in the tunnel insulating layer, mismatches in the band structure and the like are believed to be due partly to a roughness on the interface between the tunnel insulating layer and the magnetic layer.
In the case of using TMR elements for devices such as magnetic heads and MRAMs, the elements are required to have thermal stability capable of withstanding the process of manufacturing the devices. For example, heat treatment at about 200° C. to 300° C. is necessary in the manufacturing process of the elements themselves. When used for magnetic heads, the elements need to be stable at the operating environment temperature (e.g., about 120° C. to 170° C.) of the magnetic heads. Research is also carried out to fabricate MR elements on CMOSs for use as MRAM devices. Heat treatment at even higher temperatures (e.g., 400° C. to 450° C.) is necessary in the manufacturing process of CMOSs.
However, the MR properties of conventional TMR elements tend to deteriorate by heat treatment at about 300° C. to 350° C. This is presumably due to the diffusion of impurities into the tunnel insulating layer, an increase in the interface roughness and the like. The tunnel insulating layer has a very small thickness, so that it is susceptible to such effect. In order to apply TMR elements to devices such as magnetic heads and MRAMs, it is therefore important to develop TMR elements whose MR properties tend less to deteriorate when the temperature of the elements is increased by heat treatment and the like.